The present invention relates to a gasket for electric shielding having a novel internal structure or, more particularly, to a gasket composed of a matrix of an electrically insulating rubbery elastic material and an electrically conductive phase dispersed in the matrix.
When a gasket is desired to have an electrostatic or electromagnetic shielding effect, it is conventional in the prior art that the gasket is formed of a matrix or a continuum phase made of an electrically insulating rubbery elastic material and an electrically conductive phase dispersed in the matrix. Such a conductive dispersed phase is usually given by the orientation of fine filaments of a metal or carbon aligned as embedded in the matrix in a direction approximately perpendicular to the contacting surfaces of the gasket, i.e. in a direction substantially parallel with the direction of compression of the gasket when it is in use (see, for example, U.S. Pat. No. 2,885,459, No. 3,126,440, No. 3,542,939 and No. 3,708,871). Alternatively, such perpendicular conductive paths are provided by a unidirectionally arranged dispersion of electrically conductive rubber portions in the matrix (see, for example, U.S. Pat. No. 3,140,342). Further alternatively, a gasket for electric shielding is obtained by merely shaping and electrically conductive rubber prepared by incorporating a large amount of electrically conductive particles, e.g. metal powders, into a rubbery elastomer (see, for example, U.S. Pat. No. 3,609,104).
These prior art gaskets for electric shielding, however, have their respective disadvantages. In the first type of conductive filament dispersion, for example, the filament density within the surface of the gasket must be sufficiently high in order to ensure good electric conductivity so that the elastic resilience on the gasket surfaces is badly influenced and the gas-tightness for sealing is reduced when the gasket is put to use even though the electric shielding effect may be complete. In addition, the diameter and length of the filament or wire material should be accorded with the frequency of the electromagnetic waves in order to obtain optimum shielding effect. As a consequence, there may sometimes be caused surface leak current or surface discharge resulting in further deterioration or degradation of the surfaces of the gasket. Accordingly, a gasket of this type should be contacted with the sealed surfaces with an excessively large compressive force to avoid the above described drawbacks.
Gaskets of the second type, i.e. conductive paths formed with electroconductive rubbery material, are unavoidably expensive due to the complicated process for preparing them and, in addition, also require a compressive force large enough to ensure gas tightness in the contacting surfaces with the sealed surfaces.
Finally, the gaskets of the type of metal powder dispersion must be loaded with so large an amount of the metal powder to obtain good electric conductivity that the moldability as well as the mechanical properties of the gaskets are sometimes very unsatisfactory with much decreased flexibility and elasticity. Furthermore, the metal particles come off the gasket surface when rubbed or excessively compressed resulting in generation of sparks and roughened surface condition which necessitates a further increased compressive force to ensure gas thickness.
The material of the metal powder currently in use is almost exclusively limited to silver owing to the availability of very fine powders and some other reasons. Silver powders are, however, defective due to the migration of the metal particles in the presence of moisture and corrosiveness against the other metal parts if not to mention the outstanding expensiveness of silver.